Ultrasound imaging of cardiac anatomy using doppler analysis

ABSTRACT

A method includes emitting an ultrasound beam from an array of ultrasound transducers in a catheter placed in a blood pool in an organ. Echo signals reflected in response to the ultrasound beam are received in the array. Distinction is made in the echo signals between (i) first spectral signal components having Doppler shifts characteristic of blood and (ii) second spectral signal components having Doppler shifts characteristic of tissue of the organ. The first spectral signal components are suppressed relative to the second spectral signal components in the echo signals. An ultrasound image of at least a portion of the organ is reconstructed from the echo signals having the suppressed first spectral signal components. The reconstructed image is displayed to a user.

FIELD OF THE INVENTION

The present invention relates generally to medical imaging, andparticularly to Doppler ultrasound imaging using an intra-body medicalultrasound probe.

BACKGROUND OF THE INVENTION

Ultrasound Doppler imaging techniques have been previously proposed inthe art. For example, Sutherland describes noninvasive ultrasoundDoppler myocardial imaging (DMI) in a paper titled, “Colour DMI:potential applications in acquired and congenital heart disease,” ACTAPAEDIATRICA, Volume 84, Issue 410, August 1995, pages 40-48. The paperdescribes a DMI technique that allows colour Doppler imaging of cardiacstructures as opposed to blood pool imaging. This is achieved bychanging the velocity, filtering and threshold parameters of thestandard colour Doppler algorithms. DMI parameters which can be measuredare regional tissue velocity, acceleration and reflected Doppler energy.In addition, concomitant changes in the pulsed Doppler algorithms allowinterrogation of instantaneous peak velocities during the cardiac cyclein the myocardial region in which the sample volume is placed.

Although the shape of specific elements of the heart, such as an ostium,may be reconstructed using known anatomical mapping methods, suchmethods typically rely on moving a catheter to touch points on theelement. These approaches are computationally intensive and relativelytime consuming. It would be useful to have a faster mapping method, andadvantageous to have the method be non-contact.

SUMMARY OF THE INVENTION

An embodiment of the present invention that is described hereinafterprovides a method including emitting an ultrasound beam from an array ofultrasound transducers in a catheter placed in a blood pool in an organ.Echo signals reflected in response to the ultrasound beam are receivedin the array. Distinction is made in the echo signals between (i) firstspectral signal components having Doppler shifts characteristic of bloodand (ii) second spectral signal components having Doppler shiftscharacteristic of tissue of the organ. The first spectral signalcomponents are suppressed relative to the second spectral signalcomponents in the echo signals. An ultrasound image of at least aportion of the organ is reconstructed from the echo signals having thesuppressed first spectral signal components. The reconstructed image isdisplayed to a user.

In some embodiments, suppressing the first spectral signal componentsincludes filtering out the first spectral signal components from theecho signals.

In some embodiments, suppressing the first spectral signal componentsincludes attenuating the first spectral signal components in the echosignals by at least a given amount.

In an embodiment, the tissue of the organ is a wall tissue of a cardiacchamber.

In another embodiment, the method further includes focusing the emittedultrasound beam at a given blood volume and receiving in the array echosignals reflected in response to the focused ultrasound beam. In yetanother embodiment, focusing the emitted ultrasound beam includesvarying a focal length of the beam to variably collect blood Dopplershifted signals from different multiple blood volumes.

There is further provided, in accordance with another embodiment of thepresent invention, a system including a catheter including an array ofultrasound transducers and a processor. The array of ultrasoundtransducers is configured to be placed in a blood pool in an organ, toemit an ultrasound beam and to receive echo signals reflected inresponse to the ultrasound beam. The processor is configured to (a)distinguish, in the echo signals, between (i) first spectral signalcomponents having Doppler shifts characteristic of blood and (ii) secondspectral signal components having Doppler shifts characteristic oftissue of the organ, (b) suppress the first spectral signal componentsrelative to the second spectral signal components in the echo signals,(c) reconstruct an ultrasound image of at least a portion of the organfrom the echo signals having the suppressed first spectral signalcomponents, and (d) display the reconstructed image to a user.

There is furthermore provided, in accordance with another embodiment ofthe present invention, a medical imaging system, including an ultrasoundprobe and a processor. The ultrasound probe is configured for insertioninto an organ of a body, with the ultrasound probe including (i) atwo-dimensional (2D) ultrasound transducer array, and (ii) a sensorconfigured to output signals indicative of a position and orientation ofthe 2D ultrasound transducer array inside the organ. The processor isconfigured to (a) using the signals output by the sensor, registermultiple ultrasound image sections acquired by the 2D ultrasoundtransducer array, with one another, (b) produce a union of the multipleregistered ultrasound image sections, to form a rendering of at least aportion of the organ, and (c) present the rendering to a user.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, pictorial illustration of a catheter-basedultrasound imaging system using a catheter with a distal end assemblycomprising a 2D ultrasound-array and a location sensor, in accordancewith an embodiment of the present invention;

FIG. 2 is a schematic, pictorial illustration of a process for isolationof a blood Doppler-shifted component from an echo signal, followed byreconstruction of a blood-signal-free image by the system of FIG. 1 , inaccordance with an embodiment of the present invention; and

FIG. 3 is a flow chart that schematically illustrates a method ofisolation of blood Doppler-shifted components from an echo signal togenerate a filtered image using the system of FIG. 1 , in accordancewith an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Embodiments of the present invention that are described hereinafterprovide methods and systems that use a probe, such as a catheter, havinga two-dimensional (2D) array of ultrasound transducers, for producingthree-dimensional (3D) or four-dimensional (4D) ultrasound images. Inthe present context, the term “3D ultrasound image” refers to anultrasound image that represents a certain volume in three dimensions.The term “4D ultrasound image” refers to a time series of 3D ultrasoundimages of a certain volume. A 4D image can be regarded as a 3D movie,the fourth dimension being time. Another way of describing a 4D image(or rendering) is as a time-dependent 3D image (or rendering).

The 2D array produces a 3D sector-shaped ultrasound beam occupying adefined solid angle; (such a beam is referred to herein as a “wedge,” asopposed to a 1D array “fan”). The 2D array is thus able to image a 2Dsection of an inner wall of an organ, such as of a cardiac chamber.

In some embodiments, a 4D ultrasound catheter is placed in the bloodstream in proximity to an element to be mapped, such as a wall of acardiac chamber. Subsequently, a processor analyzes reflected signals(e.g., echoes) from the ultrasound wedge beam transmitted by thecatheter. Typically, the element of the heart is moving as the heartbeats, as is the blood flowing through the heart. Both movements createDoppler shifts in the frequencies of the signals received by thetransducers, but the flow of the blood, typically of the order of m/s,is significantly faster than any movement of the element being mapped.As such, the frequency shifts (Doppler shifts) in the echoes from bloodare significantly larger than the Doppler shifts in the echoes fromcardiac wall tissue. For example, for an ultrasound frequency of 5 MHz,the Doppler shift in echoes from blood is on the order of 5 kHz, whereasthe Doppler shift in echoes from cardiac wall tissue is on the order of1KHz.

The processor analyzes the Doppler-shifted signals to find the positionsand velocities of elements being imaged by the transducers. Because ofthe velocity difference between the liquid blood stream and the soft butsolid tissue element, the Doppler shift due to the blood can be easilyisolated (e.g., identified), to distinguish the surface of the elementbeing mapped. The processor suppresses the blood Doppler-shiftedcomponent of the signal. For example, the processor may digitally filterout the Doppler-shifted components to completely remove them, orattenuates the blood-related spectral components by at least a givenamount (e.g., by 20 dB). The resulting enhanced signals can then be usedto reconstruct a blood-signal-free ultrasound image of the element.

In an embodiment, a processor receives echo signals reflected inresponse to ultrasound beam emitted from an array of ultrasoundtransducers in a catheter placed in a blood pool in an organ. Theprocessor distinguishes, in the echo signals, between (i) first spectralsignal components having Doppler shifts characteristic of blood and (ii)second spectral signal components having Doppler shifts characteristicof tissue of the organ. The processor suppresses the first spectralsignal components relative to the second spectral signal components inthe echo signals. Then, the processor reconstructs an ultrasound imageof at least a portion of the organ from the echo signals having thesuppressed first spectral signal components, and displays thereconstructed image to a user

The phases of the 2D array of transducers can be electronically adjustedto focus at least a portion of the US wedge transmitted by the array ona target volume in the organ, such as a blood volume. This focusingeffect may be used to temporarily increase the quality of the signalsreflected from blood and/or from the cardiac wall, so as to enhance theDoppler measurement described above.

In some embodiments, the catheter also comprises an integral locationsensor, such as a magnetic position sensor, that is pre-registered withthe 2D array. Because of the integral location sensor, the spatialcoordinates of every voxel in the imaged section are known. Theprocessor can use the position measurements, for example, to overlay theblood-signal-free ultrasound image on another image (ultrasound orotherwise) of at least a portion of the heart.

Further to this, the processor can use the position measurements toregister multiple ultrasound image sections, acquired by the 2Dultrasound transducer array, with one another. The processor thenproduces a union of the multiple registered ultrasound image sections,to form a rendering of at least a portion of the organ, and presents therendering to a user.

In an embodiment, the processor performs the registration of themultiple ultrasound image sections while compensating for movements ofthe probe itself, or by compensating for movements due to respiration.In another embodiment, the processor produces the union by stitching themultiple ultrasound image sections one to another.

The processor can also adjust the phases of the driving signals toelectronically steer the wedge, so that the imaged element (e.g., ostiumof a pulmonary vein) is centered in a display showing the element.

System Description

FIG. 1 is a schematic, pictorial illustration of a catheter-basedultrasound imaging system 20 using a catheter 21 with a distal endassembly 40 comprising a 2D ultrasound-array 50 and a location sensor52, in accordance with an embodiment of the present invention. Integrallocation sensor 52 is pre-registered with the 2D array 50 of catheter21.

As seen, distal end assembly 40 is fitted at the distal end of a shaft22 of the catheter. Catheter 21 is inserted through a sheath 23 into aheart 26 of a patient 28 lying on a surgical table 29. The proximal endof catheter 21 is connected to a control console 24. In the embodimentdescribed herein, catheter 21 is used for ultrasound-based diagnosticpurposes, although the catheter may be further used to perform a therapysuch as electrical sensing and/or ablation of tissue in heart 26, using,for example, a tip electrode 56.

Physician 30 navigates distal end assembly 40 of catheter 21 to a targetlocation in heart 26 by manipulating shaft 22 using a manipulator 32near the proximal end of the catheter.

In an embodiment, 2D ultrasound-array 50, shown in detail in an inset25, is configured to image a left atrium of heart 26. The recordedimages are stored by processor 30 in a memory 37.

As seen in an inset 45, ultrasound array 50 comprises a 2D array 50 ofmultiple ultrasound transducers 53. Inset 45 shows ultrasound array 50navigated to an ostium wall 54 of a pulmonary vein of the left atrium.In this embodiment, 2D array 50 is an array of 32×64 US transducers. The2D array is able to image a section of the inner wall of the ostium.

Sensor 52 is configured to output signals indicative of a position,direction and orientation of the 2D ultrasound transducer array 52inside the organ. A processor of the system is configured to registermultiple ultrasound image sections, one with the other, using the signaloutput by the sensor acquired by the 2D ultrasound transducer array 50.Because of the integral location sensor, the spatial coordinates ofevery pixel in the imaged section are known.

Control console 24 comprises a processor 39, typically a general-purposecomputer, with suitable front end and interface circuits 38 for drivingultrasound transducers 53 (e.g., in a phased array manner that includessteering an ultrasound beam), and for receiving echo signals fromtransducers 53 for use by processor 39. Interface circuits 38 arefurther used for receiving signals from catheter 21, as well as for,optionally, applying treatment via catheter 21 in heart 26 and forcontrolling the other components of system 20. Console 24 also comprisesa driver circuit 34 configured to drive magnetic field generators 36.

During the navigation of distal end 22 in heart 26, console 24 receivesposition and direction signals from location sensor 52 in response tomagnetic fields from external field generators 36. Magnetic fieldgenerators 36 are placed at known positions external to patient 28,e.g., below table 29 upon which the patient is lying. These position anddirection signals are indicative of the position and direction of 2Dultrasound-array 50 in a coordinate system of the position trackingsystem.

The method of position and direction sensing using external magneticfields is implemented in various medical applications, for example, inthe CARTO™ system, produced by.

Biosense Webster, and is described in detail in U.S. Pat. Nos. 6,618,612and 6,332,089, in PCT Patent Publication WO 96/05768, and in U.S. PatentApplication Publications 2002/0065455, 2003/0120150, and 2004/0068178,whose disclosures are all incorporated herein by reference.

In some embodiments, processor 39 may be configured to operate array 52in an electronic “sweeping mode” to image a large portion of a cardiaccamber. In an embodiment, the imaged cardiac chamber (e.g., a leftatrium) is presented to physician 30 by processor 39 on a monitor 27,e.g., in as a volume rendering 55.

Processor 39 is programmed in software to carry out the functionsdescribed herein. The software may be downloaded to the computer inelectronic form, over a network, for example, or it may, alternativelyor additionally, be provided and/or stored on non-transitory tangiblemedia, such as magnetic, optical, or electronic memory.

The example configuration shown in FIG. 1 is chosen purely for the sakeof conceptual clarity. The disclosed techniques may similarly be appliedusing other system components and settings. For example, system 20 maycomprise additional components and perform non-cardiac catheterizations.

Doppler Analysis in 3D to Image Heart Anatomy

FIG. 2 is a schematic, pictorial illustration of a process for isolationof a blood Doppler-shifted component from an echo signal, followed byreconstruction of a blood-signal-free image 299 by system 20 of FIG. 1 ,in accordance with an embodiment of the present invention.

The ultrasound signals are transmitted in a form of a wedge beam 250,and echoes are detected by the same phased array 50 of catheter 21.

As seen, the mode of acquisition of 3D wedge beam 250 enablessimultaneous acquisition of a 2D image section 260 of a surface of anostium wall 54 over which blood 255 travel in a blood stream with avelocity 270, e.g., out of a pulmonary vein.

Processor 39 provides driving signals via a transducer driving unit 381of interface circuits 38 for the ultrasound transducers. A receivingunit 382 of interface circuits 38 receives the echo signals from thetransducers.

A schematic graph 275 shows an example of a spectrum of a resulting echosignal from a certain wedge. As seen, a blood Doppler-shifted component279 is well resolved from a tissue Doppler-shifted component 277.Therefore, processor 39 can isolate and remove or attenuate blood peak279, or otherwise take blood peak 279 into account in a reconstructionmodel, so as to image the geometry of section 260 with high accuracy.

In particular, this technique enables the processor to accurately imagethe boundary between the blood and the cardia wall tissue.

In an embodiment, processor 39 thus analyzes the received signals (e.g.,received echoes) from the transducers via receiving unit 382 ofinterface circuits 38 to:

(i) identify and filter out Doppler-shifted blood components from asignal, and, optionally, to derive position and velocity of bloodelements 290; and

(ii) generate an image 299 of soft tissue (e.g., of cardiac wallsurfaces) using the blood-signal filtered signals (i.e., echo signalswithout blood Doppler-shifted components) Processor 39 saves the aboveinformation in memory 37.

The processor may filter out or attenuate Doppler-shifted bloodcomponent 279 using digital filtration applied in various different waysto digital echo signals that were digitized by unit 382. For example, inone embodiment, the processor may apply frequency-domain filtering thatremoves signal components having Doppler shifts corresponding to bloodvelocity, and retains signal components having Doppler shiftscorresponding to cardiac-wall velocity. In another embodiment, theprocessor defines, in the frequency domain, one range corresponding toblood velocity and another (lower-frequency) range corresponding tocardiac-wall velocity. The processor then suppresses the spectral rangerelated to blood. Note that for this kind of filtering, the processormay use, for example, simple threshold comparison and may not need toidentify any spectral peaks in the signal.

In yet another embodiment, the processor identifies only the bloodcomponent in order to remove it, without identifying the wall-tissuecomponent. In a further embodiment, the processor identifies the slowwall-related spectral component, and suppress other signals.

As noted above, processor 39 may adjust the relative phases of thedriving signals provided to the 2D array of transducers to focus atleast a portion 280 of ultrasound wedge beam 250 transmitted by thearray onto a blood volume 284. This focusing effect may be used toenhance the Doppler measurement described above. Additionally oralternatively, for example at a slightly different time, the processormay focus portion 280 on wall tissue surface area 288, to furtherenhance the Doppler imaging technique.

In an embodiment, to emit an ultrasound beam focused in a blood volume,the processor varies a focal length of the beam to collect aDoppler-shifted blood component from a location within this bloodvolume. By varying the locations, a blood velocity profile can becharacterized by the processor, for example, over a path in bloodbetween the catheter and the wall surface. The resulting blood velocityprofile may be used in more elaborated (e.g., spatially weighted)removal of Doppler-shifted blood components of signals.

FIG. 3 is a flow chart that schematically illustrates a method ofisolation of blood Doppler-shifted components from an echo signal togenerate a filtered image 299 using system 20 of FIG. 1 , in accordancewith an embodiment of the present invention.

The process begins in positioning 4D ultrasound (US) catheter 21 in theblood stream in proximity to cardiac wall tissue region to be imaged,such as near ostium wall tissue 54 of FIG. 1 , at a catheter placementstep 302.

Next, processor 39 commands the emission of wedge ultrasound (U/S) beam250 by catheter 21, by applying driving signals to 2D-array 50, usingunit 381, at US emission step 303.

In a return signal acquisition step 304, a reflected US signal isacquired by processor 39 using array 50 and unit 382.

Processor 39 analyzes reflected US signals to identify differentDoppler-shifted components (e.g., components 277 and 279 of FIG. 2 ) ofthe echo signals, at signal analysis step 306.

Then, processor 39 filters out blood Doppler-shifted components fromreflected signals, at blood signal filtration out step 308.

Using the blood-signal filtered out signals, processor 39 reconstructs aUS image, such as image 299, of ostium wall tissue 54, at a wall surfaceimage reconstruction step 310.

At a displaying step 312, processor 39 displays the reconstructed imageon monitor 27.

A reviewer, such as physician 30, can decide (314) if the blood-signalfiltered US image is of sufficient quality, and save the image in memory37, at image saving step 316.

If physician 30 deems (314) that the quality of the image is not goodenough, then, based on his/her input, processor 39 focuses a portion ofthe emitted US beam on blood volume, at US beam focusing step 318, toobtain an improved acquisition of blood signal, and the process returnsto step 303.

The flow chart of FIG. 3 is brought purely by way of example for thesake of conceptual clarity. For example, using blood signals to obtainblood information 290 is omitted for simplicity. As another example, theprocessor may additionally electronically steer wedge 250 duringacquisition.

Although the embodiments described herein mainly address cardiacapplications, the methods and systems described herein can also be usedin other body organs. For example, the disclosed technique can be usedwith visualizing large blood vessels of the body, such as located in theabdomen and the brain.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsub-combinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art. Documents incorporated by reference inthe present patent application are to be considered an integral part ofthe application except that to the extent any terms are defined in theseincorporated documents in a manner that conflicts with the definitionsmade explicitly or implicitly in the present specification, only thedefinitions in the present specification should be considered.

1. A method, comprising: emitting an ultrasound beam from an array ofultrasound transducers in a catheter placed in a blood pool in an organ;receiving in the array echo signals reflected in response to theultrasound beam; distinguishing, in the echo signals, between (i) firstspectral signal components having Doppler shifts characteristic of bloodand (ii) second spectral signal components having Doppler shiftscharacteristic of tissue of the organ; suppressing the first spectralsignal components relative to the second spectral signal components inthe echo signals; reconstructing an ultrasound image of at least aportion of the organ from the echo signals having the suppressed firstspectral signal components; and displaying the reconstructed image to auser.
 2. The method according to claim 1, wherein suppressing the firstspectral signal components comprises filtering out the first spectralsignal components from the echo signals.
 3. The method according toclaim 1, wherein suppressing the first spectral signal componentscomprises attenuating the first spectral signal components in the echosignals by at least a given amount.
 4. The method according to claim 1,wherein the tissue of the organ is a wall tissue of a cardiac chamber.5. The method according to claim 1, and comprising focusing the emittedultrasound beam at a given blood volume and receiving in the array echosignals reflected in response to the focused ultrasound beam.
 6. Themethod according to claim 5, wherein focusing the emitted ultrasoundbeam comprises varying a focal length of the beam to variably collectblood Doppler shifted signals from different multiple blood volumes. 7.A medical imaging method, comprising: inserting an ultrasound probe intoan organ of a body, the ultrasound probe comprising: a two-dimensional(2D) ultrasound transducer array; and a sensor configured to outputsignals indicative of a position and orientation of the 2D ultrasoundtransducer array inside the organ; using the signals output by thesensor, registering multiple ultrasound image sections acquired by the2D ultrasound transducer array, with one another; producing a union ofthe multiple registered ultrasound image sections, to form a renderingof at least a portion of the organ; and presenting the rendering to auser.
 8. The method according to claim 7, wherein registering themultiple ultrasound image sections comprises compensating for movementsof the probe.
 9. The method according to claim 7, wherein registeringthe multiple ultrasound image sections comprises compensating formovements due to respiration.
 10. The method according to claim 7,wherein producing the union comprises stitching the multiple ultrasoundimage sections one to another.
 11. A system, comprising: a cathetercomprising an array of ultrasound transducers, the array configured tobe placed in a blood pool in an organ, to emit an ultrasound beam and toreceive echo signals reflected in response to the ultrasound beam; and aprocessor, which is configured to: distinguish, in the echo signals,between (i) first spectral signal components having Doppler shiftscharacteristic of blood and (ii) second spectral signal componentshaving Doppler shifts characteristic of tissue of the organ; suppressthe first spectral signal components relative to the second spectralsignal components in the echo signals; reconstruct an ultrasound imageof at least a portion of the organ from the echo signals having thesuppressed first spectral signal components; and display thereconstructed image to a user.
 12. The system according to claim 11,wherein the processor is configured to suppress the first spectralsignal components by filtering out the first spectral signal componentsfrom the echo signals.
 13. The system according to claim 11, wherein theprocessor is configured to suppress the first spectral signal componentsby attenuating the first spectral signal components in the echo signalsby at least a given amount.
 14. The system according to claim 11,wherein the tissue of the organ is a wall tissue of a cardiac chamber.15. The system according to claim 11, wherein the array is furtherconfigured to focus the emitted ultrasound beam at a given blood volumeand receive echo signals reflected in response to the focused ultrasoundbeam.
 16. The system according to claim 15, wherein the array isconfigured to focus the emitted ultrasound beam by varying a focallength of the beam to variably collect blood Doppler shifted signalsfrom different multiple blood volumes.
 17. A medical imaging system,comprising: an ultrasound probe configured for insertion into an organof a body, the ultrasound probe comprising: a two-dimensional (2D)ultrasound transducer array; and a sensor configured to output signalsindicative of a position and orientation of the 2D ultrasound transducerarray inside the organ; and a processor, which is configured to: usingthe signals output by the sensor, register multiple ultrasound imagesections acquired by the 2D ultrasound transducer array, with oneanother; produce a union of the multiple registered ultrasound imagesections, to form a rendering of at least a portion of the organ; andpresent the rendering to a user.
 18. The system according to claim 17,wherein, in registering the multiple ultrasound image sections, theprocessor is configured to compensate for movements of the probe. 19.The system according to claim 17, wherein, in registering the multipleultrasound image sections, the processor is configured to compensate formovements due to respiration.
 20. The system according to claim 17,wherein the processor is configured to produce the union by stitchingthe multiple ultrasound image sections one to another.